This invention relates to an apparatus and method for reducing agglomerates to a predetermined particle size. Such a process is generally distinguished from processes akin to pulverizing both in the manner in which the reduction of particle size takes place and in the end product. The process to which this invention relates is sometimes referred to as grinding or milling. The purpose of these processes is to arrive at a product having a relatively uniform microfine particle size.
Many different types of agglomerates are processed in this type of machine. Examples include paint and ink pigments, barium, manganese, zinc ferrite and, in the food processing industry, products such as coffee and cocoa.
There are two basic prior art processes for grinding or milling agglomerates to reduce their particle size.
One of these processes uses an apparatus variously referred to as an annular chamber mill, grinding mill, agitator mill or peg mill. Development of this type of machine began with a hollow cylinder rotating on a horizontal axis and loaded with a certain amount of material to be ground and a certain quantity of grinding elements--most commonly small spherical steel balls but sometimes other hard substances such as walnut shells. The rotation of the cylinder causes the balls to tumble over and collide with each other in layers, grinding the material as the balls collide with each other. Typically, the loads were fed into the cylinder and ground for a period of six to sixty hours, depending on the size of the agglomerate and the desired size of the ground particles.
Next, vertical and horizontal agitator mills were developed by using a slow-running agitator to keep the ball charge moving. As with the horizontal tube mill, the vertical agitator mill operated periodically. During operation, a pump drew off the ground material at the bottom of the mill and pumped it back in at the top to make sure that it was uniformly processed.
More recently, particular attention has been paid to the shape and operation of the design of the agitators which keeps the ball charge activated. It was recognized that two out of the three factors which influence the grinding effect can be easily held constant. These are the size of the contact surface and the mass of the grinding balls. Therefore, the speed of the agitator arms, which sets the grinding elements in motion can be made to vary and thereby control operating parameters. Through development it was determined that the most efficient way of keeping the balls in motion is by the use of pegs which rotate through the ball charge. This led to the development of the annular chamber mill or peg mill of current design.
In current-style peg mills, the agitator is designed as a hollow shaft with a relatively large diameter. Pegs project outwardly from the shaft. The pegs are kept relatively short to limit the difference in speed between the hollow shaft and the extreme ends of the pegs. These pegs are arranged in rings on the hollow shaft, one above the other at relatively short intervals.
Also located in rings around the inside walls of the grinding tank are counter-pegs which fit between the pegs circulating on the agitator shaft. The purpose of the counter-pegs is to make sure that the grinding elements are thoroughly activated. Since the diameter of the grinding tank is relatively large and since up to eight agitator pegs are arranged in one annular element, the agitator shaft can be driven at a slow speed. The pegs are typically manufactured from material such as tungsten carbide in an effort to reduce wear. Nevertheless, the massive amounts of friction created between the material being ground, the grinding elements and the pegs generate a tremendous amount of heat and wear out the moving parts at a relatively rapid rate even at very slow speed. Furthermore, the physics involved in explaining the manner in which material is ground in peg mills is a complex one. In some cases, grinding is carried out most effectively in the lower part of the grinding tank. In other instances, grinding occurs most effectively in the top of the grinding tank or somewhere in between, depending on the throughput and the viscosity of the material being ground. Care must therefore be taken to control the throughput rate and the grinding rate to insure that the mill does not overheat and that the material is completely and uniformly ground
To deal with the heat generated in peg mills, cooling jackets are provided which encircle the periphery of the grinding tank and also extend upwardly through the hollow shaft on which the inner grinding pegs are located. Cold water is circulated through these jackets in order to maintain temperature within the grinding tank within established limits. However, uniform cooling in this type of apparatus is impossible. As is apparent, the most efficient cooling will take place at the surface of the cooling jackets. Therefore, product which is in contact with the cooling jackets will be cooled to a very substantial degree, while material in the middle of the ball charge and between the inner and outer cooling jackets will be cooled to a much lower degree, if at all. Since some types of materials, such as some paint pigments and food products, are quickly affected by heat, it is apparent that the relatively efficient cooling next to the cooling jackets and the relatively inefficient cooling in the middle of the grinding area of the grinding tank has the potential for creating an end product which is an average of individual particles having distinctly different characteristics due to heat variation.
To summarize, a state-of-the-art peg mill such as described above provides continuous throughput with a relatively high output of ground product. These advantages are off-set by difficulty in uniformly cooling the product being ground and a relatively uneven particle size in relation to other types of grinding. Finally, the ball mill is very difficult to clean because of the vast amount of surface area contained on the balls, the pegs and the shaft and grinding tank walls. Therefore, it is quite often necessary to use a particular mill for only a single substance. This is particularly true when grinding pigments for paints where contamination of pigments of one color by pigments of another color results in an unusable end product.
The second type of grinding mill is known as a roller mill. In this type of mill, grinding takes place by shearing the material between closely spaced-apart cylindrical rolls rotating at different speeds. This type of mill cannot be used for continuous throughput but, rather, is used to grind a single charge of product. At the beginning of the process, the rolls are slightly spaced-apart. The charge of material to be ground is applied to the rolls and spread on the rolls until an even coating covers all of the rolls. Then, the rolls are moved together. The differential speed of rotation of the rolls causes the product to be ground through the shearing action of one roll against the material coating the adjacent roll.
A roller mill offers the advantage of very uniform cooling since a very thin film of material is always in contact with the surface of the roll which is cooled by circulating water or another heat absorbing medium. Also, the rolls of the roller mill are relatively easy to clean in relation to the peg mill described above. Therefore, the roller mill can be cleaned and used to grind a variety of substances, including different paint and ink pigments. While the roller mill offers these advantages, it also cannot be used for continuous throughput of material and has a relatively low rate of material output. In addition, the roller mill is open to the user and therefore dangerous since fingers, hair, neckties, etc. can be caught in the rapidly rotating rolls.
The two types of mills described above have been used for many years in spite of the fact that they both offer significant disadvantages. In many cases, a choice of which of the two mills to use depends on which disadvantages are acceptable and are most easily compensated for by their respective advantages.
The invention described in this application eliminates all of the disadvantages described above for the roller mill and the peg mill while retaining their advantages. Specifically, the invention described below is a continuous throughput mill which, like the roller mill described above, achieves very efficient cooling of the product. For this reason, a very high output of ground product can be achieved. The apparatus is easy to clean and produces an output material having a very even particle size.